The alt.usage.english FAQ

"sirloin"/"baron of beef"

(Word Origins)

"Sirloin" comes from Old French _surlonge_, from _sur_ "above"
and _loigne_ "loin". Its current spelling may have been influenced
by a story that a King of England (variously said to be Henry VIII,
James I, and Charles II) "knighted" this cut of beef because of
its superiority.
A "baron of beef" is a joint consisting of two sirloins left
uncut at the backbone. This "baron" may have originated as a joke
on "sirloin", or it may be an independent word.